1. Field of the Invention
This invention relates to the preparation of polyurethane-polyurea compositions and is more particularly concerned with improvements in a process of preparing polyurethane-polyureas by reaction of organic polyisocyanates, polyols and aromatic diamine extenders and with the compositions so prepared.
2. Description of the Prior Art
U.S. Pat. No. 4,218,543 provides a comprehensive review of the prior art relating to the preparation of polyurethane-polyureas by the reaction of organic polyisocyanates, polymeric polyols and various polyamine extenders and said review is incorporated herein by reference. The same patent also describes a significant advance in the art by teaching how to make this type of composition using reaction injection molding (RIM) techniques. The key factors in the method there described are the choice of a particular group of aromatic diamines as the extender and the use of a "one-shot" procedure, i.e. a procedure in which the organic polyisocyanate, polymeric polyol and amine extenders are brought together simultaneously and allowed to react, as opposed to a "prepolymer" procedure in which two or more of the reactants are prereacted before being brought into contact with the remainder of the reactants.
The particular aromatic diamines targeted by the above reference are those aromatic diamines having alkyl groups in both positions ortho to one amino group and in at least one of the positions ortho to the second amino group. A particularly preferred group of aromatic diamines is represented by 1-methyl-3,5-diethyl-2,4-diaminobenzene, 1-methyl-3,5-diethyl-2,6-diaminobenzene and mixtures thereof. These compounds are those derived by diethylation of 2,4- and or 2,6-toluenediamine or mixtures thereof and are generally referred to by the initials "DETA."
The reference also requires that the organic polyisocyanate be 4,4'-methylenebis(phenyl isocyanate) or liquid modifications thereof derived by reacting the diisocyanate with a minor amount of a low molecular weight diol or by converting a portion of the diisocyanate to carbodiimide in accordance with known procedures; see the above cited U.S. Pat. No. 4,218,543 at Column 7, lines 17-28.
The method of the reference provides a means of producing polyurethane-polyureas by a RIM type process using reaction mixtures which have gel times of as low an order as about 3 seconds but which, in spite of the shortness of the latter time, can still be employed to fill complicated mold cavities having a relatively large volume. Further, the physical properties, including structural strength properties, of the resulting moldings are generally excellent.
We have found, however, that the use of the one-shot procedure described in the reference has an inherent disadvantage which arises from the need to maintain the diisocyanate separate from the polymeric polyol and the diamine extender and to feed the diisocyanate as a separate stream to the mixing head, the second stream comprising a mixture of the polymeric polyol and diamine. Since the equivalent weight of the diisocyanate is much lower than that of the polymeric polyol the proportion by weight in which the diisocyanate stream and the combined polyol and diamine stream are brought together in the mixing head is substantially less than equal, the diisocyanate stream proportion being only approximately one-half of that of the other stream. As will be obvious to one skilled in the art, it is generally desirable that the two streams being fed to the high pressure mixing heads used in RIM processing be mixed in substantially equal amounts by volume in order to achieve the maximum output per unit of time from the RIM head. In view of the above noted restriction on the relative amounts in which the two streams are to be mixed in the one-shot process discussed above, it is apparent that there is a significant limitation built into the maximum output of mixed reactants which can be derived by the process in question. This limitation on output, when taken together with the very short gel times of the reaction mixture produced, places very severe limits on the total weight of reactants which can be introduced into any given mold using the process of the reference. This translates into a clear limit on the size of any article which can be produced in a single shot using the process.
We have now found that these various limitations can be overcome by utilizing the diisocyanate in the form of certain prepolymers formed from certain polymeric polyols. As will be apparent from the detailed disclosure below, this enables one to achieve a RIM process in which the two streams being fed to the mixing head are mixed in a wide range of proportions including close to equal proportions, thereby enabling one to operate at much higher outputs per unit time than is the case with the reference procedure. Further, it is found that the use of the prepolymer perceptibly increases the gel time of the reaction mixes and permits a greater degree of latitude to the operator than is available using the extremely short reaction times which characterize the one-shot procedure.
Our findings are all the more unexpected in view of the previous experience in the use of aromatic diamines as extenders for isocyanate-terminated prepolymers. U.S. Pat. No. 3,428,610 discloses the preparation of cross-linked polyurethanes by reaction of isocyanate-terminated prepolymers with the very same class of aromatic diamines which the aforesaid U.S. Pat. No. 4,218,543 indicates is the preferred class for use in the one-shot procedure discussed above. However, the U.S. Pat. No. 3,428,610 patent reports processing times varying over a range between 3 and 45 minutes, all of which would be unacceptable in a RIM procedure operated commercially. In contrast, utilizing the particular prepolymers and procedures described below, we have found that gel times as low as 2.5 seconds can be achieved.